1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and, more specifically, to an improved apparatus for chemical-mechanical polishing (CMP).
2. Background
Nonplanar surfaces, when present in integrated circuits having complex, high density multilevel interconnections, may cause the optical resolution of photolithographic processing steps to be poor, which could inhibit the printing of high density lines. Non-planar layer surfaces degrade the ability to control linewidths during the photolithographic process. It is thus important, when making such complex integrated circuits, to planarize the surface of many of the layers that make up the integrated circuit.
Various techniques have been developed to planarize certain layers formed during the process of making integrated circuits. In one approach, known as chemical-mechanical polishing (CMP), protruding steps, such as those that may be formed along the upper surface of interlayer dielectrics ("ILDs"), are removed by polishing. Chemical-mechanical polishing may also be used to planarize conformally deposited metal layers to form planar plugs or vias.
CMP may also be used to planarize layers that are typically much thinner than ILDs, for example, polysilicon layers that are used to form gate electrodes or interconnect in Metal Oxide Semiconductor (MOS) processes. Unlike CMP planarization of ILDs where many thousands of Angstroms of material are removed, the deposited thickness of the polysilicon layer in a modern MOS process is typically on the order of a few thousand Angstroms.
CMP generally uses a slurry, which is introduced onto a polishing pad, to achieve the removal of a portion of the surface being polished. When the desired amount of material removal has been achieved, the polished surface must be cleaned to remove contaminants, such as excess slurry particles, that can adversely affect the microelectronic component and interconnect structures that are to be formed subsequent to a particular CMP step.
In a typical CMP system, as shown in FIG. 1, a wafer 102 is placed face down on a rotating table 104 covered with a polishing pad 106, which has been coated with a slurry 108. A carrier 100, which can be made of a thick, substantially rigid metal plate 114 that is attached to a rotatable shaft 105, is used to apply a downward force against the backside of wafer 102. A retaining ring 117 is used to center wafer 102 onto carrier 100 and to prevent wafer 102 from slipping laterally. Typically, the surface of wafer 102 extends outwardly beyond the polishing side surface (i.e., the wear surface) of retaining ring 117. A resilient carrier pad 112 positioned between metal plate 114 and wafer 102, is typically used to press against the backside of wafer 102. Often, plate 114 will be manufactured with a slight convex curvature so as to bend the central portion of a wafer outward. By applying the downward force, and rotating wafer 102, while simultaneously rotating slurry covered pad 106 for a selected amount of time, a desired amount of material may be removed from the upper surface of a thin film such that the surface of wafer 102 is planarized.
Retaining ring 117 is also referred to in the art as a wafer retaining ring, or a wear ring. The polishing side surface of retaining ring 117 is also referred to as the wear surface.
FIG. 2 provides a top view of a conventional CMP system, showing polishing pad 202, retaining ring 204, and slurry delivery arm 206.
During the CMP process, material is removed not only from the surface of the wafer being planarized, but also from the wear surface of retaining ring 117. Moreover, retaining ring 117 tends to wear, sometimes unevenly, thus introducing a source of non-uniformity and non-repeatability into the CMP process. Consequently, retaining ring 117 must be replaced and/or refurbished frequently to maintain the desired level of uniformity and repeatability. Unfortunately, it is a costly process to maintain the wear surface of retaining ring 117 because, the polishing tool must be taken off-line while the carrier is disassembled, the worn ring removed and a new ring installed. Similarly, even though retaining ring 117 may be machined several times to overcome the uneven wearing that results from use, this is also costly because retaining ring 117 is typically made from a very expensive plastic such as polyphenylene sulfide (PPS).
Accordingly, there is a need for CMP methods and apparatus that reduce the cost of semiconductor manufacturing.